Digestion of sewage sludge



NOV. 27, 1934. A. J, FlsCHER D IoEs'rIoN oF sEwAGE SLUDGE Filed oct.i1', 1930 Patented Nov. 27, 1934 UNITED STATES 1,982,246 DIGESTION FASEWAGE SLUDGE Anthony J. Fischer, New York, N. Y., assignor, by mesneassignments, to The Dorr Company, Inc., New York, N. Y., a corporationof Delaware Application October 11,

4 Claims.

This invention relates to the digestion of sewage sludge and its objectis to concentrate the sewage sludge before it is digested. I attain thisobject by causing the production of, as an inter- 5 mediate step in theusual process, an acid condition in the thin sludge by exposing it tothe action of septic bacteria between the clarifier and the digester.

Solids which settle very slowly in the clarifier,

l0 settle to about one-third of their volume when in the presence of anacid condition. So, after separation by decantation, the solids which goto the digester have eliminated largely'therefrom, the large volume ofliquid which has heretofore l5 resulted in the objectionable overflowfrom the digester.

In ordinary plant practice, activated sludge is pumped directly from thefinal clarifier to the digester. In a well operated plant, the solidcontent of this sludge rarely exceeds 1.5% by weight. This high moisturecontent is detrimental in that it increases the volume of digesteroverflow and causes greater heat loss in a heated digester. The thinsludge also requires a larger sized digester.

If, however, the sludge is pumped to a thickening tank of the Dorr typewherein mechanical means are provided forfacilitating the compression ofthe sludge, for instance, as shown in Patent No. 1,938,934, patentedDecember 12, 1933, and allowed to remain there for a period ofapproximately24 hours, a much denser sludge is obtained. This sludge canthen be digested. The

' thickening action appears to be due in great part to septic actionwhich takes place when the sludge is allowed to stand for long periods.`It is also aided by the greater zone of compression because of thegreater permissible depth of sludge in the separate tank.

NhvA feature of this invention is to detain the sludge from a clarifierbefore its digestion, in a decanting thickener while acid bacterialconditions exist in the thickener. 'I'he thickener has travelling rakingmembers which are effective to squeeze out or release acid liquid fromthe sludge, causing it to escape upwardly and out through the thickeneroverflow.

In order that my use of this thickening tank may be clearly understood,I have illustrated its hook-up by an accompanying flow-sheet which makesit plain that my acid conditioning tank or thickener is placed betweenthe usual secondary or final clarifier and the digester. Sludge from thethickener goes to the digester and overflow from the decanting thickeneris returned to the usual aeration tank. I

In the fiowsheet illustrated in the drawing, the raw sewage flows to theaeration tank where it is mixed with air and the mixed liquid flows to asecondary clarifier. 60 clarifier is led to the thickener which embodiesThe underflow from the 1930, serial No. 487,999 (c1. 21o-s) A myinvention. In this thickener the sewage sludge is settled andconcentrated. It is inth'is thickener that its bulk or volume ismaterially decreased. This compressed sludge is drawn out as underflowfrom the thickenerand fed to the usual sewage digester. As is usual insewage layouts, some eiliuent or overflow from the thickener v time ofretention of the sewage in secondary clarifier before adding to thedigester.

The advantage of this system is that the usual size of the digester canbe greatly reduced; the heat losses lessened; and the volume of overflowmaterially cut down.

Whereas I have described my acid conditioning tankV for use withactivated sludge, I may use it in connection with thickening raw sludgeor with digester overflow. y

I claim:` i c c 1. A sewage sludge treating apparatus comprising anaeration tank, a secondary clarifier into which sludge from the aerationtank is adapted to pass, an acid-conditioning decanting thickener forreceiving sludge from the clarifier, and a digester yfor receivingsludge from the thickener. l

2. The process of sewage treatment comprising exposing the sewage toprimary clariiication to separate sludge therefrom, exposing theeffluent from said primary clarification to aeration, exposing theaerated sewage to secondary clarification to separate sludge therefrom,and then exposing the separated sludge to digestion; characterized bythickening the sludge or sludges from the clarification steps anddecanting liquid therefrom prior to digestion.

3. The process of sewage treatment comprising clarifying aerated sewageto separate solids therefrom in the form of sludge, thickening saidlfeparated sludge for a time while decanting liquid therefrom, and thendigesting said thickened sludge.

4. In the process of sewage treatment comprising clarifying sewage toseparate solids therefrom'y in the form of sludge, and subsequentlydigesting the sludge, the step of detaining the sludge from theclarifier, before its digestion, in a decanting thickener wherein acidbacterial conditions exist. ANTHONY J. FISCHER.

